Fabricating High Performance, DonorAcceptor Copolymer Solar Cells by Spray-Coating in Air

We report the fabrication of high performance organic solar cells by spray-coating the photoactive layer in air. The photovoltaic blends consist of a blend of carbazole and benzothiadiazole based donoracceptor copolymers and the fullerene derivative PC70BM. Here, we formulate a number of photovoltaic inks using a range of solvent systems that we show can all be deposited by spray casting. We use a range of techniques to characterize the structure of such films, and show that spray-cast films have comparable surface roughness to spin-cast films and that vertical stratification that occurs during film drying reduces the concentration of PCBM towards the underlying PEDOT:PSS interface. We also show that the active layer thickness and the drying kinetics can be tuned through control of the substrate temperature. High power conversion efficiencies of 4.3%, 4.5% and 4.6% were obtained for solar cells made from a blend of PC70BM with the carbazole-based co-polymers PCDTBT, P2 and P1. By applying a low temperature anneal after the deposition of the cathode, the efficiency of spray-cast solar-cells based on a P2:PC70BM blend is increased to 5.0%. Spray coating holds significant promise as a technique capable of fabricating large-area, high performance organic solar cells in air.